Most common boatlifts consist of a cradle supported by four cables mounted to a top beam with a gear unit, which drives a pipe thereby winding the cable and lifting the boat. Further, in the most common water-lifts for boats, a boat cradle is swing-mounted on two pairs of parallel link units which are in turn swing-mounted on a base frame. A hydraulic cylinder unit, comprising a lift cylinder, piston, and piston rod, is mounted between the base frame and one of the parallel link units so that extension of the piston rod from the lift cylinder causes the parallel link units and cradle to swing upwardly. The lift cylinder is supplied with pressurized water from any convenient source to raise the lift and is lowered by emptying the cylinder.
In the past, the boat cradle has usually been held in position by one or the other of two arrangements, and namely, by raising the parallel linkage past center (past a vertical position) to engage a stop, or by latching the lift cylinder unit such that its piston rod cannot retract relative to its cylinder. Such latching has been accomplished by a dog swing-mounted on the upper end of the lift cylinder and arranged to swing down by gravity into locking engagement with a stop when the desired elevation of the boat cradle has been reached. However, no attempt to stabilize the cradle against forces caused by high winds or other phenomena has ever been adequately produced.